Copy iso8601/xmlschema impl from time gem

This is a temporary move until we can follow CRuby and implement
a native version of this function.

See jruby#8476

Author: headius

core/src/main/java/org/jruby/Ruby.java

@@ -562,11 +562,6 @@ private Ruby(RubyInstanceConfig config) {
                 getLoadService().require("subspawn/replace-builtin");
             }
         }
-
-        // FIXME: How should this be loaded as it is not really stdlib but depends on stdlib to be loaded.
-        try {
-            getLoadService().require("time");
-        } catch (LoadError e) {} // work-around failed classpath only test (which must be omitting stdlib somehow)
     }
 
     private void initProfiling() {

core/src/main/ruby/jruby/kernel.rb

@@ -29,3 +29,4 @@ module JRuby
 load 'jruby/kernel/thread.rb'
 load 'jruby/kernel/data.rb'
 load 'jruby/kernel/integer.rb'
+load 'jruby/kernel/time.rb'

core/src/main/ruby/jruby/kernel/time.rb

@@ -0,0 +1,35 @@
+class Time
+  # From stdlib time library, see https://github.com/jruby/jruby/issues/8476
+  unless method_defined?(:xmlschema)
+    #
+    # Returns a string which represents the time as a dateTime defined by XML
+    # Schema:
+    #
+    #   CCYY-MM-DDThh:mm:ssTZD
+    #   CCYY-MM-DDThh:mm:ss.sssTZD
+    #
+    # where TZD is Z or [+-]hh:mm.
+    #
+    # If self is a UTC time, Z is used as TZD.  [+-]hh:mm is used otherwise.
+    #
+    # +fraction_digits+ specifies a number of digits to use for fractional
+    # seconds.  Its default value is 0.
+    #
+    #     require 'time'
+    #
+    #     t = Time.now
+    #     t.iso8601  # => "2011-10-05T22:26:12-04:00"
+    #
+    # You must require 'time' to use this method.
+    #
+    def xmlschema(fraction_digits=0)
+      fraction_digits = fraction_digits.to_i
+      s = strftime("%FT%T")
+      if fraction_digits > 0
+        s << strftime(".%#{fraction_digits}N")
+      end
+      s << (utc? ? 'Z' : strftime("%:z"))
+    end
+  end
+  alias iso8601 xmlschema unless method_defined?(:iso8601)
+end