Update httparty requirement from = 0.18.1 to = 0.21.0

Updates the requirements on [httparty](https://github.com/jnunemaker/httparty) to permit the latest version.
- [Release notes](https://github.com/jnunemaker/httparty/releases)
- [Changelog](https://github.com/jnunemaker/httparty/blob/master/Changelog.md)
- [Commits](jnunemaker/httparty@v0.18.1...v0.21.0)

---
updated-dependencies:
- dependency-name: httparty
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -3,7 +3,7 @@ PATH
   specs:
     ruby-pardot (1.4.1)
       crack (= 0.4.3)
-      httparty (= 0.18.1)
+      httparty (= 0.21.0)
 
 GEM
   remote: https://rubygems.org/
@@ -12,12 +12,10 @@ GEM
       safe_yaml (~> 1.0.0)
     diff-lcs (1.4.4)
     fakeweb (1.3.0)
-    httparty (0.18.1)
-      mime-types (~> 3.0)
+    httparty (0.21.0)
+      mini_mime (>= 1.0.0)
       multi_xml (>= 0.5.2)
-    mime-types (3.3.1)
-      mime-types-data (~> 3.2015)
-    mime-types-data (3.2020.1104)
+    mini_mime (1.1.2)
     multi_xml (0.6.0)
     rspec (3.5.0)
       rspec-core (~> 3.5.0)

ruby-pardot.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |s|
   s.rubyforge_project         = "ruby-pardot"
 
   s.add_dependency 'crack', '0.4.3'
-  s.add_dependency 'httparty', '0.18.1'
+  s.add_dependency 'httparty', '0.21.0'
 
   s.add_development_dependency "bundler", ">= 1.10"
   s.add_development_dependency "rspec", "3.5.0"