Raise an error if a response is recorded with an invalid body.

This can occur when a non-standard Faraday stack is used, where a response-modifying middleware comes after the HTTP adapter.

For vcr#159.

lib/vcr/structs.rb

@@ -60,6 +60,11 @@ def try_encode_string(string, encoding)
 
       def initialize(*args)
         super
+
+        if body && !body.is_a?(String)
+          raise ArgumentError, "#{self.class} initialized with an invalid body: #{body.inspect}."
+        end
+
         # Ensure that the body is a raw string, in case the string instance
         # has been subclassed or extended with additional instance variables
         # or attributes, so that it is serialized to YAML as a raw string.

spec/vcr/structs_spec.rb

@@ -49,6 +49,12 @@
   it 'converts nil to a blank string' do
     instance(nil).body.should eq("")
   end
+
+  it 'raises an error if given another type of object as the body' do
+    expect {
+      instance(:a => "hash")
+    }.to raise_error(ArgumentError)
+  end
 end
 
 module VCR