Fix broken URI joining for `bamboo_url` with suffixes

If one had configured a `bamboo_url` like http://foo.bar/bamboo in the
previous implementation the plugin directed it's request i.e. to
http://foo.bar/rest/... instead of http://foo.bar/bamboo/rest/...

`URI.join` only works correctly, if the prefix URL has
  - at least one or more  trailing '/'
  - the appended parts are _not_ prefixed with '/'

The current implementation should work with all sorts of Bamboo base URLs.
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

@@ -38,6 +38,7 @@ v 8.9.0 (unreleased)
   - Add option to project to only allow merge requests to be merged if the build succeeds (Rui Santos)
   - Fix issues filter when ordering by milestone
   - Added artifacts:when to .gitlab-ci.yml - this requires GitLab Runner 1.3
+  - Bamboo Service: Fix missing credentials & URL handling when base URL contains a path (Benjamin Schmid)
   - Todos will display target state if issuable target is 'Closed' or 'Merged'
   - Fix bug when sorting issues by milestone due date and filtering by two or more labels
   - Add support for using Yubikeys (U2F) for two-factor authentication

app/models/project_services/bamboo_service.rb

@@ -59,7 +59,7 @@ class BambooService < CiService
   end
 
   def build_info(sha)
-    url = URI.join(bamboo_url, "/rest/api/latest/result?label=#{sha}").to_s
+    url = URI.join("#{bamboo_url}/", "rest/api/latest/result?label=#{sha}").to_s
 
     if username.blank? && password.blank?
       @response = HTTParty.get(url, verify: false)
@@ -78,11 +78,11 @@ class BambooService < CiService
 
     if @response.code != 200 || @response['results']['results']['size'] == '0'
       # If actual build link can't be determined, send user to build summary page.
-      URI.join(bamboo_url, "/browse/#{build_key}").to_s
+      URI.join("#{bamboo_url}/", "browse/#{build_key}").to_s
     else
       # If actual build link is available, go to build result page.
       result_key = @response['results']['results']['result']['planResultKey']['key']
-      URI.join(bamboo_url, "/browse/#{result_key}").to_s
+      URI.join("#{bamboo_url}/", "browse/#{result_key}").to_s
     end
   end
 
@@ -111,7 +111,7 @@ class BambooService < CiService
     return unless supported_events.include?(data[:object_kind])
 
     # Bamboo requires a GET and does take authentification
-    url = URI.join(bamboo_url, "/updateAndBuild.action?buildKey=#{build_key}").to_s
+    url = URI.join("#{bamboo_url}/", "updateAndBuild.action?buildKey=#{build_key}").to_s
 
     if username.blank? && password.blank?
       HTTParty.get(url, verify: false)