Fix failed parsing on multiple required arguments

Fixes #25.

tests/Language/GraphQL/AST/LexerSpec.hs

@@ -71,8 +71,8 @@ spec = describe "Lexer" $ do
             parse float "" "-1.123e4567" `shouldParse` (-1.123e4567)
 
         it "lexes punctuation" $ do
-            parse bang "" "!" `shouldParse` '!'
-            parse dollar "" "$" `shouldParse` '$'
+            parse bang "" "!" `shouldParse` "!"
+            parse dollar "" "$" `shouldParse` "$"
             runBetween parens `shouldSucceedOn` "()"
             parse spread "" "..." `shouldParse` "..."
             parse colon "" ":" `shouldParse` ":"

tests/Language/GraphQL/AST/ParserSpec.hs

@@ -24,3 +24,9 @@ spec = describe "Parser" $ do
         parse document "" `shouldSucceedOn` [r|{
               hello(text: "Argument")
             }|]
+
+    it "accepts two required arguments" $
+        parse document "" `shouldSucceedOn` [r|
+            mutation auth($username: String!, $password: String!){
+                test
+            }|]