Hash request URIs before using them as a cache key

Issue #132 notes that the caching middleware can generate keys
which are too long when the input is a complex URL, leading to
errors like
`Errno::ENAMETOOLONG: File name too long @ rb_sysopen`. (The
effective length limit depends on what cache driver you're using.)

At the moment, we use the normalised request URI as the cache key.
The fixes the issue by using `Digest::SHA1.hexdigest` to hash
the URLs, giving us a constant 20-byte result.

The trade-off of using a hash is that:

* there is the potential for collisions (as rare as they will
realistically be!)
* performing a hash takes time

lib/faraday_middleware/response/caching.rb

@@ -1,5 +1,6 @@
 require 'faraday'
 require 'forwardable'
+require 'digest/sha1'
 # fixes normalizing query strings:
 require 'faraday_middleware/addressable_patch' if defined? ::Addressable::URI
 
@@ -66,7 +67,8 @@ def cache_key(env)
         url.query = params.any? ? build_query(params) : nil
       end
       url.normalize!
-      url.request_uri
+
+      Digest::SHA1.hexdigest(url.request_uri)
     end
 
     def params_to_ignore

spec/unit/caching_spec.rb

@@ -82,7 +82,7 @@
     let(:options) { {:write_options => {:expires_in => 9000 } } }
 
     it "passes on the options when writing to the cache" do
-      expect(@cache).to receive(:write).with("/",
+      expect(@cache).to receive(:write).with(Digest::SHA1.hexdigest("/"),
                                              instance_of(Faraday::Response),
                                              options[:write_options])
       get('/')
@@ -92,7 +92,7 @@
       let(:options) { {} }
 
       it "doesn't pass a third options parameter to the cache's #write" do
-        expect(@cache).to receive(:write).with("/", instance_of(Faraday::Response))
+        expect(@cache).to receive(:write).with(Digest::SHA1.hexdigest("/"), instance_of(Faraday::Response))
         get('/')
       end
     end