Fix BitbucketServer::Client failing with Webmock 3.5.1

Webmock 3.1.0 changed the behavior to return `nil` for the body if an
HTTP 204 No Content response were received
(https://github.com/bblimke/webmock/commit/b837e642782501a6904d0cb1aad391dd6f49ada3).

Update the Bitbucket Server connection to ignore these No Content
response codes.

lib/bitbucket_server/connection.rb

@@ -77,6 +77,7 @@ module BitbucketServer
     private
 
     def check_errors!(response)
+      return if ActionDispatch::Response::NO_CONTENT_CODES.include?(response.code)
       raise ConnectionError, "Response is not valid JSON" unless response.parsed_response.is_a?(Hash)
 
       return if response.code >= 200 && response.code < 300

spec/lib/bitbucket_server/client_spec.rb

@@ -64,7 +64,7 @@ describe BitbucketServer::Client do
     let(:url) { "#{base_uri}rest/api/1.0/projects/SOME-PROJECT/repos/my-repo/branches" }
 
     it 'requests Bitbucket to create a branch' do
-      stub_request(:post, url).to_return(status: 204, headers: headers, body: '{}')
+      stub_request(:post, url).to_return(status: 204, headers: headers, body: nil)
 
       subject.create_branch(project, repo_slug, branch, sha)
 
@@ -78,7 +78,7 @@ describe BitbucketServer::Client do
     let(:url) { "#{base_uri}rest/branch-utils/1.0/projects/SOME-PROJECT/repos/my-repo/branches" }
 
     it 'requests Bitbucket to create a branch' do
-      stub_request(:delete, url).to_return(status: 204, headers: headers, body: '{}')
+      stub_request(:delete, url).to_return(status: 204, headers: headers, body: nil)
 
       subject.delete_branch(project, repo_slug, branch, sha)