Proper checking of enum with non zero default

proto2 syntax allows the first enum to have a non zero value. This means any
field using that default has a non zero default without having an explicit
default being set. So when deciding what runtime info is needed, don't rely
on an explicit default, always check that the values aren't zero.

Fixes https://github.com/google/protobuf/issues/1453

objectivec/Tests/GPBMessageTests.m

diff --git a/objectivec/Tests/GPBMessageTests.m b/objectivec/Tests/GPBMessageTests.m
index 4354615..0779c5a 100644
--- a/objectivec/Tests/GPBMessageTests.m
+++ b/objectivec/Tests/GPBMessageTests.m
@@ -1947,4 +1947,11 @@
                  EnumTestMsg_MyEnum_NegTwo);
 }
 
+- (void)testOneBasedEnumHolder {
+  // Test case for https://github.com/google/protobuf/issues/1453
+  // Message with no explicit defaults, but a non zero default for an enum.
+  MessageWithOneBasedEnum *enumMsg = [MessageWithOneBasedEnum message];
+  XCTAssertEqual(enumMsg.enumField, MessageWithOneBasedEnum_OneBasedEnum_One);
+}
+
 @end

objectivec/Tests/unittest_objc.proto

diff --git a/objectivec/Tests/unittest_objc.proto b/objectivec/Tests/unittest_objc.proto
index 9483cb1..6bcfbf7 100644
--- a/objectivec/Tests/unittest_objc.proto
+++ b/objectivec/Tests/unittest_objc.proto
@@ -401,3 +401,13 @@
 
   repeated MyEnum mumble = 4;
 }
+
+// Test case for https://github.com/google/protobuf/issues/1453
+// Message with no explicit defaults, but a non zero default for an enum.
+message MessageWithOneBasedEnum {
+  enum OneBasedEnum {
+    ONE = 1;
+    TWO = 2;
+  }
+  optional OneBasedEnum enum_field = 1;
+}