Raise the correct exception in fast_serialize_string

[eregon]

Fix: #634

• Related to String#to_json behavior different for pure vs native ext #344

[byroot]

You can conditionally run it with if defined?(JSON::Pure).

I'll be trying to make implementations converge over time, but short term it's helpful to test the divergent behavior.

[eregon]

OK, done.

[byroot]

Actually there is already a test for it, so just remove the commented out code I think:

json/test/json/json_generator_test.rb

Lines 448 to 456 in f887daa

	if defined?(JSON::Ext::Generator) and RUBY_PLATFORM != "java"
	def test_valid_utf8_in_different_encoding
	utf8_string = "€™"
	wrong_encoding_string = utf8_string.b
	# This behavior is historical. Not necessary desirable. We should deprecated it.
	# The pure and java version of the gem already don't behave this way.
	assert_equal utf8_string.to_json, wrong_encoding_string.to_json
	assert_equal JSON.dump(utf8_string), JSON.dump(wrong_encoding_string)
	end

[eregon]

That's different, it's for valid UTF-8 bytes, this is invalid UTF-8 bytes (both cases using the BINARY encoding on the String).
Also that test only checks the C ext behavior, and this tests the pure-Ruby behavior.