avoid changing existing environment keys

lib/zero/request/parameter.rb

@@ -12,6 +12,8 @@ module Zero
       ENV_KEY_QUERY   = 'QUERY_STRING'
       # the key for the payload
       ENV_KEY_PAYLOAD = 'rack.input'     
+      # the key for custom parameters
+      ENV_KEY_CUSTOM = 'zero.params.custom'
       # the key for the content type
       ENV_KEY_CONTENT_TYPE = 'CONTENT_TYPE'
       # all content types which used for using the body as a parameter input
@@ -40,8 +42,12 @@ module Zero
       def initialize(environment)
         @query   = extract_query_params(environment)
         @payload = extract_payload_params(environment)
-        @custom  = {}
-        environment['zero.params.custom'] = @custom
+        if environment.has_key?(ENV_KEY_CUSTOM)
+          @custom = environment[ENV_KEY_CUSTOM]
+        else
+          @custom  = {}
+          environment[ENV_KEY_CUSTOM] = @custom
+        end
       end
 
       # get a parameter

spec/unit/request/parameter/initialize_spec.rb

@@ -0,0 +1,12 @@
+require 'spec_helper'
+
+describe Zero::Request::Parameter, '#initialize' do
+  subject { Zero::Request::Parameter.new(env) }
+  let(:env) { EnvGenerator.get('/get', {
+    'zero.params.custom' => {'foo' => 'bar'} }) }
+
+  it 'does not overwrite parameters' do
+    subject
+    expect(env['zero.params.custom']).to have_key('foo')
+  end
+end