Raise error for duplicate map<> keys when decoding TextFormat

[thomasvl]

From the Map Features section of the proto2 syntax spec.

When parsing from the wire or when merging, if there are duplicate map keys the last key seen is used. When parsing a map from text format, parsing may fail if there are duplicate keys.

The second sentence there is the key. Right now Swift doesn't do this, but there seems to be some rumblings that this might get fixed in other languages and added to the conformance test.