v1.2.0

Notable changes:

• #605 jsonpb.Unmarshal correctly handles escaped strings for some of the well-known types
• #627 jsonpb.Unmarshal correctly returns an error when trying to marshal a scalar enum into repeated enums.
• #630 jsonpb.Unmarshal correctly handles string versions of numeric primitives
• #644 jsonpb.Unmarshal correctly handles enum values in maps
• #624 fixes a performance regression where nested maps led to quadratic behavior
• #628 reverts strict UTF-8 validation for proto2 string fields
• #660 makes UTF-8 validation errors for proto3 string fields non-fatal