Adds support of empty string field_name. (#141)

amazon/ion/writer_binary.py

@@ -168,7 +168,7 @@ def intern_symbol(sym):
     def intern_symbols(event):
         field_name = event.field_name
         annotations = event.annotations
-        if field_name:
+        if field_name or field_name == '':
             event = event.derive_field_name(intern_symbol(field_name))
         if annotations:
             event = event.derive_annotations(

tests/test_simpleion.py

@@ -129,6 +129,18 @@ def bytes_of(*args, **kwargs):
             [IonPyDict.from_value(IonType.STRUCT, {}), ],
             _Expected(b'\xD0', b'{}')
         ),
+        (
+            [{u'': u''}, ],
+            _Expected(
+                bytes_of([
+                    0xDE,  # The lower nibble may vary. It does not indicate actual length unless it's 0.
+                    VARUINT_END_BYTE | 2,  # Field name 10 and value 0 each fit in 1 byte.
+                    VARUINT_END_BYTE | 10,
+                    0x80    # Empty string
+                ]),
+                b"{'':\"\"}"
+            )
+        ),
         (
             [{u'foo': 0}, ],
             _Expected(