Optionally raise on empty hash filter definitions

I've recently spent several hours debugging a problem that was caused by an empty hash definition
```ruby
required do
   hash :hash_name
end
```

Such definition discards any hash content being passed in, which is very
confusing and I reckon rarely useful, but very unintuitive interface
(found other 3 bugs in the codebase caused by the exact same problem).

This change makes such definitions raise, but only if
Mutations.raise_on_empty_hash_filter is set to true (defualt false).
Such optional raising avoids breaking change.

Overall, I recommend changing the interface to raise on bogus definition
as a default, instead of the opt-in.

lib/mutations.rb

@@ -41,7 +41,16 @@ def cache_constants=(val)
     def cache_constants?
       @cache_constants
     end
+
+    def raise_on_empty_hash_filter=(val)
+      @raise_on_empty_hash_filter = val
+    end
+
+    def raise_on_empty_hash_filter
+      @raise_on_empty_hash_filter
+    end
   end
 end
 
 Mutations.cache_constants = true
+Mutations.raise_on_empty_hash_filter = false

lib/mutations/hash_filter.rb

@@ -23,6 +23,10 @@ def initialize(opts = {}, &block)
       @required_inputs = {}
       @current_inputs = @required_inputs
 
+      if Mutations.raise_on_empty_hash_filter && !block_given?
+        raise ArgumentError.new("Hash parameter can't be created without passing the block")
+      end
+
       if block_given?
         instance_eval(&block)
       end

spec/hash_filter_spec.rb

@@ -105,6 +105,22 @@ def input.to_hash
     assert_equal ({"baz" => :integer}), errors.symbolic
   end
 
+  it "allows empty hash definitions" do
+    Mutations::HashFilter.new
+    # the fact that we're here means it didn't raise
+  end
+
+  describe "with raise_on_empty_hash_filter=true" do
+    it "raises for empty hash definition" do
+      old_value = Mutations.raise_on_empty_hash_filter
+      Mutations.raise_on_empty_hash_filter = true
+
+      assert_raises(ArgumentError) { Mutations::HashFilter.new }
+    ensure
+      Mutations.raise_on_empty_hash_filter = old_value
+    end
+  end
+
   describe "optional params and nils" do
     it "bar is optional -- it works if not passed" do
       hf = Mutations::HashFilter.new do
@@ -167,7 +183,7 @@ def input.to_hash
       assert_equal ({"foo" => "bar"}), filtered
       assert_equal nil, errors
     end
-    
+
     it "bar is optional -- discards empty if it needs to be stripped" do
       hf = Mutations::HashFilter.new do
         required do
@@ -182,7 +198,7 @@ def input.to_hash
       assert_equal ({"foo" => "bar"}), filtered
       assert_equal nil, errors
     end
-    
+
     it "bar is optional -- don't discard empty if it's spaces but stripping is off" do
       hf = Mutations::HashFilter.new do
         required do