fix: use t.stringLiteral to create an identifier (#92)

From #89
`Identifiers can't have quotes. If the key is supposed to be a string, it should be t.stringLiteral(name). Because it is an identifier it is an invalid AST.`

Author: pbarbiero

src/createObjectExpression.js

@@ -32,7 +32,7 @@ const createObjectExpression = (t: BabelTypes, object: InputObjectType): ObjectE
 
     properties.push(
       t.objectProperty(
-        t.identifier('\'' + name + '\''),
+        t.stringLiteral(name),
         newValue
       )
     );