Prevent uses of node reference from being assigned default attributes if already declared.

Scenario:
```dot
z;
node [gender=male]; y;
node [gender=female]; x;
z -- x;
z -- y;
```
Intent for z to have no (or empty-valued) gender attribute, as specified in para 1, "Lexical and Semantic Notes" of the
DOT Language Specification.

Previous behaviour would assign z gender=female.
Merge pull request #1 from edgemaster/master

Prevent uses of node reference from being assigned default attributes if already declared.