Clarify numeric casting behavior for blank strings [ci-skip]

`ActiveModel::Type::Decimal`, `ActiveModel::Type::Float`, and
`ActiveModel::Type::Integer` cast blank strings to `nil` rather than
casting them with `to_d`, `to_f`, or `to_i`.  This commit clarifies that
behavior in each type's documentation.

Author: jonathanhefner

activemodel/lib/active_model/type/decimal.rb

@@ -13,15 +13,24 @@ module Type
     #     attribute :weight, :decimal
     #   end
     #
+    # Numeric instances are converted to BigDecimal instances. Any other objects
+    # are cast using their +to_d+ method, except for blank strings, which are
+    # cast to +nil+. If a +to_d+ method is not defined, the object is converted
+    # to a string using +to_s+, which is then cast using +to_d+.
+    #
     #   bag = BagOfCoffee.new
-    #   bag.weight = "0.0001"
     #
-    #   bag.weight # => 0.1e-3
+    #   bag.weight = 0.01
+    #   bag.weight # => 0.1e-1
     #
-    # Numeric instances are converted to BigDecimal instances. Any other objects
-    # are cast using their +to_d+ method, if it exists. If it does not exist,
-    # the object is converted to a string using +to_s+, which is then coerced to
-    # a BigDecimal using +to_d+.
+    #   bag.weight = "0.01"
+    #   bag.weight # => 0.1e-1
+    #
+    #   bag.weight = ""
+    #   bag.weight # => nil
+    #
+    #   bag.weight = :arbitrary
+    #   bag.weight # => nil (the result of `.to_s.to_d`)
     #
     # Decimal precision defaults to 18, and can be customized when declaring an
     # attribute:

activemodel/lib/active_model/type/float.rb

@@ -13,18 +13,24 @@ module Type
     #     attribute :weight, :float
     #   end
     #
+    # Values are cast using their +to_f+ method, except for the following
+    # strings:
+    #
+    # - Blank strings are cast to +nil+.
+    # - <tt>"Infinity"</tt> is cast to <tt>Float::INFINITY</tt>.
+    # - <tt>"-Infinity"</tt> is cast to <tt>-Float::INFINITY</tt>.
+    # - <tt>"NaN"</tt> is cast to <tt>Float::NAN</tt>.
+    #
     #   bag = BagOfCoffee.new
-    #   bag.weight = "0.25"
     #
+    #   bag.weight = "0.25"
     #   bag.weight # => 0.25
     #
-    # Values are coerced to their float representation using their +to_f+
-    # methods. However, the following strings which represent floating point
-    # constants are cast accordingly:
+    #   bag.weight = ""
+    #   bag.weight # => nil
     #
-    # - <tt>"Infinity"</tt> is cast to <tt>Float::INFINITY</tt>.
-    # - <tt>"-Infinity"</tt> is cast to <tt>-Float::INFINITY</tt>.
-    # - <tt>"NaN"</tt> is cast to <tt>Float::NAN</tt>.
+    #   bag.weight = "NaN"
+    #   bag.weight # => Float::NAN
     class Float < Value
       include Helpers::Numeric
 

activemodel/lib/active_model/type/integer.rb

@@ -11,13 +11,17 @@ module Type
     #     attribute :age, :integer
     #   end
     #
+    # Values are cast using their +to_i+ method, except for blank strings, which
+    # are cast to +nil+. If a +to_i+ method is not defined or raises an error,
+    # the value will be cast to +nil+.
+    #
     #   person = Person.new
-    #   person.age = "18"
     #
+    #   person.age = "18"
     #   person.age # => 18
     #
-    # Values are cast using their +to_i+ method, if it exists. If it does not
-    # exist, or if it raises an error, the value will be cast to +nil+:
+    #   person.age = ""
+    #   person.age # => nil
     #
     #   person.age = :not_an_integer
     #   person.age # => nil (because Symbol does not define #to_i)