babel-generator: keep double quotes in StringLiteral for JSX parent

[hzoo]

Fixes https://phabricator.babeljs.io/T6859

Idea is to check if the parent node of the StringLiteral is of type JSX - although maybe we can just check t.isJSXAttribute?

var quotes = '!';

React.createClass({
  render() {
+    return <View multiple="attributes" attribute="Value's got a single quote in it" />;
-    return <View multiple='attributes' attribute='Value\'s got a single quote in it' />;
  }
});

@steveluscher

[codecov-io]

Current coverage is 84.79%

Merging #3189 into master will decrease coverage by -0.31% as of 98cf473

@@            master   #3189   diff @@
======================================
  Files          215     215       
  Stmts        15605   15605       
  Branches      3334    3334       
  Methods          0       0       
======================================
- Hit          13281   13233    -48
- Partial        677     724    +47
- Missed        1647    1648     +1

Review entire Coverage Diff as of 98cf473

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