Allow strings to be parsed into enums in variables

Fixes #17.

src/executor_tests/enums.rs

@@ -96,7 +96,7 @@ fn does_not_accept_string_literals() {
 #[test]
 fn accepts_strings_in_variables() {
     run_variable_query(
-        "{ toString(color: RED) }",
+        "query q($color: Color!) { toString(color: $color) }",
         vec![
             ("color".to_owned(), InputValue::string("RED")),
         ].into_iter().collect(),

src/macros/enums.rs

@@ -94,7 +94,7 @@ macro_rules! graphql_enum {
 
         impl $crate::FromInputValue for $name {
             fn from(v: &$crate::InputValue) -> Option<$name> {
-                match v.as_enum_value() {
+                match v.as_enum_value().or_else(|| v.as_string_value()) {
                     $(
                         Some(graphql_enum!(@as_pattern, $ename))
                             => Some(graphql_enum!(@as_expr, $eval)), )*

src/types/utilities.rs

@@ -1,7 +1,7 @@
 use std::collections::HashSet;
 use ast::InputValue;
 use schema::model::{SchemaType, TypeType};
-use schema::meta::{MetaType, InputObjectMeta};
+use schema::meta::{MetaType, InputObjectMeta, EnumMeta};
 
 pub fn is_valid_literal_value(schema: &SchemaType, arg_type: &TypeType, arg_value: &InputValue) -> bool {
     match *arg_type {
@@ -20,6 +20,13 @@ pub fn is_valid_literal_value(schema: &SchemaType, arg_type: &TypeType, arg_valu
             }
         }
         TypeType::Concrete(t) => {
+            // Even though InputValue::String can be parsed into an enum, they
+            // are not valid as enum *literals* in a GraphQL query.
+            match (arg_value, arg_type.to_concrete()) {
+                (&InputValue::String(_), Some(&MetaType::Enum(EnumMeta { .. }))) => return false,
+                _ => ()
+            }
+
             match *arg_value {
                 ref v @ InputValue::Null |
                 ref v @ InputValue::Int(_) |