Fix graphql-tag/loader by properly stringifying GraphQL source (#65)

[Loc.prototype.toJson](https://github.com/graphql/graphql-js/blob/77f2785e6c740a95e7615dad41e4ac1b663cea0e/src/language/parser.js#L1048-L1050) will never return the `source` key (which holds the stringified version of the query that we need when using the Webpack loader. We'll manually add it since the reference to `source` still exists on `doc.loc`

loader.js

@@ -41,7 +41,10 @@ function expandImports(source, doc) {
 module.exports = function(source) {
   this.cacheable();
   const doc = gql`${source}`;
-  const outputCode = `var doc = ${JSON.stringify(doc)};`;
+  const outputCode = `
+    var doc = ${JSON.stringify(doc)};
+    doc.loc.source = ${JSON.stringify(doc.loc.source)};
+  `;
   const importOutputCode = expandImports(source, doc);
 
   return outputCode + "\n" + importOutputCode + "\n" + `module.exports = doc;`;

test.js

@@ -20,6 +20,23 @@ const assert = require('chai').assert;
       assert.equal(gql`{ field(input: "${0}") }`.kind, 'Document');
     });
 
+    it('allows interpolation of documents generated by the webpack loader', () => {
+      const sameFragment = "fragment SomeFragmentName on SomeType { someField }";
+
+      const jsSource = loader.call(
+        { cacheable() {} },
+        "fragment SomeFragmentName on SomeType { someField }"
+      );
+      const module = { exports: undefined };
+      eval(jsSource);
+
+      const document = gql`query { ...SomeFragmentName } ${module.exports}`;
+      assert.equal(document.kind, 'Document');
+      assert.equal(document.definitions.length, 2);
+      assert.equal(document.definitions[0].kind, 'OperationDefinition');
+      assert.equal(document.definitions[1].kind, 'FragmentDefinition');
+    });
+
     it('parses queries through webpack loader', () => {
       const jsSource = loader.call({ cacheable() {} }, '{ testQuery }');
       const module = { exports: undefined };