Fixed-width == and === docs do not work with RDoc short-hand

```
$ echo '+foo+' | rdoc --pipe
<p><code>foo</code></p>

$ echo '+==+' | rdoc --pipe
<p>+==+</p>
```

Author: zzak

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -58,9 +58,9 @@ module ClassMethods
         #   semantically equal to be considered different.
         #   For instance two hashes with the same keys and values but a different order have a
         #   different serialized representation, but are semantically equal once deserialized.
-        #   If set to +true+ the comparison will be done on the deserialized object. This options
-        #   should only be enabled if the +type+ is known to have a proper +==+ method that deeply
-        #   compare the objects.
+        #   If set to +true+ the comparison will be done on the deserialized object.
+        #   This options should only be enabled if the +type+ is known to have
+        #   a proper <tt>==</tt> method that deeply compare the objects.
         # * +yaml+ - Optional. Yaml specific options. The allowed config is:
         #   * +:permitted_classes+ - +Array+ with the permitted classes.
         #   * +:unsafe_load+ - Unsafely load YAML blobs, allow YAML to load any class.

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -37,7 +37,7 @@ def self.references(attributes)
 
     # Define how a class is converted to Arel nodes when passed to +where+.
     # The handler can be any object that responds to +call+, and will be used
-    # for any value that +===+ the class given. For example:
+    # for any value that <tt>===</tt> the class given. For example:
     #
     #     MyCustomDateRange = Struct.new(:start, :end)
     #     handler = proc do |column, range|