Simple check for symbol keys in schemas

I didn't want to iterate over the entire schema and I think checking the
first key should catch most errors.

Fixes #9

lib/json_schemer.rb

@@ -12,6 +12,7 @@ module JSONSchemer
   class UnsupportedMetaSchema < StandardError; end
   class UnknownRef < StandardError; end
   class InvalidFileURI < StandardError; end
+  class InvalidSymbolKey < StandardError; end
 
   DRAFT_CLASS_BY_META_SCHEMA = {
     'http://json-schema.org/draft-04/schema#' => Schema::Draft4,

lib/json_schemer/schema/base.rb

@@ -36,6 +36,7 @@ def initialize(
         keywords: nil,
         ref_resolver: DEFAULT_REF_RESOLVER
       )
+        raise InvalidSymbolKey, 'schemas must use string keys' if schema.is_a?(Hash) && schema.first.first.is_a?(Symbol)
         @root = schema
         @format = format
         @formats = formats

test/json_schemer_test.rb

@@ -390,6 +390,17 @@ def test_it_handles_json_strings
     assert !schema.valid?('1')
   end
 
+  def test_it_checks_for_symbol_keys
+    assert_raises(JSONSchemer::InvalidSymbolKey) { JSONSchemer.schema({ :type => 'integer' }) }
+    schema = JSONSchemer.schema(
+      { '$ref' => 'http://example.com' },
+      :ref_resolver => proc do |uri|
+        { :type => 'integer' }
+      end
+    )
+    assert_raises(JSONSchemer::InvalidSymbolKey) { schema.valid?(1) }
+  end
+
   def test_cached_ref_resolver
     schema = {
       'properties' => {