Remove unnecessary conversions (#315)

Found with github.com/mdempsky/unconvert.

	/Users/tamird/src/go/src/github.com/golang/protobuf/jsonpb/jsonpb.go:625:32: unnecessary conversion
	/Users/tamird/src/go/src/github.com/golang/protobuf/proto/encode.go:177:30: unnecessary conversion
	/Users/tamird/src/go/src/github.com/golang/protobuf/proto/encode.go:181:26: unnecessary conversion
	/Users/tamird/src/go/src/github.com/golang/protobuf/proto/text_parser.go:868:21: unnecessary conversion
diff --git a/proto/text_parser.go b/proto/text_parser.go
index 61f83c1..5e14513 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -865,7 +865,7 @@
 		return p.readStruct(fv, terminator)
 	case reflect.Uint32:
 		if x, err := strconv.ParseUint(tok.value, 0, 32); err == nil {
-			fv.SetUint(uint64(x))
+			fv.SetUint(x)
 			return nil
 		}
 	case reflect.Uint64: