Improve MathMan url construction

Fixes: CNVS-38278

Originally we were using Addressable::URI#merge to build up the url,
merge in Addressable isn't like merge in most URL parsers, instead of
handling merging the path component it just clobbers it. Instead, we
want to use #join which exhibits the behavior we want.

Test Plan:
* Configure Math Man to use a URL with a path component
* Rendering equations should work

Change-Id: I6f62daaf4dc2a36e39c8f00d2a974c3ba4032c3f
Reviewed-on: https://gerrit.instructure.com/119741
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Tucker McKnight <tmcknight@instructure.com>
Product-Review: Tyler Pickett <tpickett@instructure.com>

lib/math_man.rb

@@ -19,7 +19,7 @@ require "addressable/uri"
 
 module MathMan
   def self.url_for(latex:, target:)
-    uri = base_url.merge(path: "/#{target}")
+    uri = base_url.join(target.to_s)
     uri.query = "tex=#{latex}"
     uri.to_s
   end
@@ -45,7 +45,9 @@ module MathMan
 
     def base_url
       with_plugin_settings do |plugin_settings|
-        Addressable::URI.parse(plugin_settings[:base_url])
+        Addressable::URI.parse(plugin_settings[:base_url]).tap do |uri|
+          uri.path << '/' unless uri.path.end_with?('/')
+        end
       end
     end
 

spec/lib/math_man_spec.rb

@@ -21,7 +21,8 @@ describe MathMan do
   let(:latex) do
     '\sqrt{25}+12^{12}'
   end
-  let(:service_url) { 'http://www.mml-service.com' }
+  # we explicitly don't want a trailing slash here for the url tests
+  let(:service_url) { 'http://www.mml-service.com/beta' }
   let(:use_for_mml) { false }
   let(:use_for_svg) { false }
 
@@ -41,6 +42,12 @@ describe MathMan do
   end
 
   describe '.url_for' do
+    it 'must retain the path from base_url setting' do
+      url = MathMan.url_for(latex: latex, target: :mml)
+      parsed = Addressable::URI.parse(url)
+      expect(parsed.path).to eq ('/beta/mml')
+    end
+
     it 'should include target string in generated url' do
       expect(MathMan.url_for(latex: latex, target: :mml)).to match(/mml/)
       expect(MathMan.url_for(latex: latex, target: :svg)).to match(/svg/)