Assert the proper status code for PUT

See https://tools.ietf.org/html/rfc7231#section-4.3.4

Money quote:

    If the target resource does not have a current representation and the
    PUT successfully creates one, then the origin server MUST inform the
    user agent by sending a 201 (Created) response.  If the target
    resource does have a current representation and that representation
    is successfully modified in accordance with the state of the enclosed
    representation, then the origin server MUST send either a 200 (OK) or
    a 204 (No Content) response to indicate successful completion of the
    request.

api_spec.rb

@@ -61,7 +61,7 @@ def check_dir_listing_content_type(content_type)
     end
 
     it "works" do
-      [200, 201].must_include @res.code
+      @res.code.must_equal 201
       @res.headers[:etag].wont_be_nil
       @res.headers[:etag].must_be_etag
     end
@@ -101,7 +101,7 @@ def check_dir_listing_content_type(content_type)
 
     it "works" do
       # before runs for each block once, so we have to put all blocks into one
-      [200, 201].must_include @res.code
+      [204, 200].must_include @res.code
       @item_etag.wont_be_nil
       @item_etag.must_be_etag