Do not raise Error upon invalid content-length

See: #365 (comment)

lib/http/response.rb

@@ -88,13 +88,12 @@ def flush
 
     # Value of the Content-Length header
     #
-    # @raise [HTTP::Error] if presented Content-Type value is not integer
     # @return [Integer, nil]
     def content_length
       value = @headers[Headers::CONTENT_LENGTH]
       Integer(value) if value
     rescue ArgumentError
-      raise Error, "Invalid value of Content-Type: #{value.inspect}"
+      nil
     end
 
     # Parsed Content-Type header

spec/lib/http/response_spec.rb

@@ -40,12 +40,9 @@
       it { is_expected.to eq 5 }
     end
 
-    context "with Content-Length not an integer" do
+    context "with invalid Content-Length" do
       let(:headers) { {"Content-Length" => "foo"} }
-
-      it "raises an error" do
-        expect { subject }.to raise_error(HTTP::Error)
-      end
+      it { is_expected.to be_nil }
     end
   end