[7392] fix conformance test in PHP JSON parser

[antongrbin]

Motivation

This PR fixes failing JSON conformance tests for php with name IgnoreUnknownEnumStringValue*.

The JSON parsing spec was discussed in https://github.com/protocolbuffers/protobuf/issues/7392.

Recent similar changes in other languages:

• Python: https://github.com/protocolbuffers/protobuf/commit/86abf35ef5ee5b1004ec11bebb36d84c2ef6645e
• Swift: https://github.com/apple/swift-protobuf/pull/1345
• C#: https://github.com/protocolbuffers/protobuf/pull/15758
• C++: https://github.com/protocolbuffers/protobuf/pull/16479

[antongrbin]

@jskeet, since you worked with me on the linked issue https://github.com/protocolbuffers/protobuf/issues/7392, is it OK to ask you to help me find a PHP reviewer to move this forward?

[bshaffer]

@antongrbin I can take a look

[antongrbin]

@bshaffer thank you for the review offer && polite ping :)

[antongrbin]

@bshaffer friendly ping :)