Update JSON unknown field support (apple#771)

* Support erroring on unknown fields during JSON parse.

According to the spec:
  https://developers.google.com/protocol-buffers/docs/proto3#json_options
Unknown field name encountered while parsing JSON should cause
errors, but language can provide a option to allow those fields
to be skipped instead. We had it backwards, and always skipped
them with no way to get an error.

- Add an option to ignore unknown fields during JSON decode.
- Add a JSON Decoding Error to indicated when an unknown
  field was encountered.
- Wire up the option and new error.
- Update the existing tests get the behaviour they arelady
  required around unknown fields in JSON.

* Add a specific test around JSON unknown field handing.

- Ensure the unknown field stop parsing (so any other errors don't show.)
- Ensure ensure the unknown field name is reported.

* Fix typo in two test field names.

These were actually unknown field name getting skipped and not
real test of setting the field to null.

* Update the generated_swift_names_* files.

* Support the conformance test's new ignore_unknown_json.

* Bump the version number to 1.1.0

Since we are adding a minor behavior change around unknown field names
in JSON parsing, bump the minor version to help call out this subtle
change in behavior.

Fixes apple#769

Author: thomasvl

Protos/generated_swift_names_enum_cases.proto

@@ -257,6 +257,7 @@ message GeneratedSwiftReservedEnums {
   enum hasValue { NONE_hasValue = 0; }
   enum hour { NONE_hour = 0; }
   enum i { NONE_i = 0; }
+  enum ignoreUnknownFields { NONE_ignoreUnknownFields = 0; }
   enum index { NONE_index = 0; }
   enum init { NONE_init = 0; }
   enum inout { NONE_inout = 0; }