Do a shallow copy of params hash in Notifier#build_notice (airbrake#234)

At various points in the filter call chains of building up an Airbrake
notice, the code mutates the params hash that is passed in (adding
keys like :priority and safe_level). A simple dup means the calling
code no longer has to worry about that params hash mutating.

lib/airbrake-ruby/notifier.rb

@@ -74,7 +74,7 @@ def build_notice(exception, params = {})
         exception[:params].merge!(params)
         exception
       else
-        Notice.new(@config, convert_to_exception(exception), params)
+        Notice.new(@config, convert_to_exception(exception), params.dup)
       end
     end
 

spec/notifier_spec.rb

@@ -522,6 +522,13 @@ def to_json(*)
     end
 
     context "given a non-exception with calculated internal frames only" do
+      it "prevents mutation of passed-in params hash" do
+        params = { only_this_item: true }
+        notice = @airbrake.build_notice(RuntimeError.new('bingo'), params)
+        notice[:params][:extra_item] = :not_in_original_params
+        expect(params).to eq(only_this_item: true)
+      end
+
       it "returns the internal frames nevertheless" do
         backtrace = [
           "/airbrake-ruby/lib/airbrake-ruby/notifier.rb:84:in `build_notice'",