Be more tolerant of existing env vars during tests

The download cache tests can fail if certain environment variables
exist on the users machine. This commit makes those tests more tolerant
of this.

[#101130824]

Author: Christopher Frost

spec/java_buildpack/util/cache/download_cache_spec.rb

@@ -172,7 +172,7 @@
 
   context do
 
-    let(:environment) { { 'http_proxy' => 'http://proxy:9000' } }
+    let(:environment) { { 'http_proxy' => 'http://proxy:9000', 'HTTP_PROXY' => nil } }
 
     it 'uses http_proxy if specified' do
       stub_request(:get, uri)
@@ -189,7 +189,7 @@
 
   context do
 
-    let(:environment) { { 'HTTP_PROXY' => 'http://proxy:9000' } }
+    let(:environment) { { 'HTTP_PROXY' => 'http://proxy:9000', 'http_proxy' => nil } }
 
     it 'uses HTTP_PROXY if specified' do
       stub_request(:get, uri)
@@ -206,7 +206,7 @@
 
   context do
 
-    let(:environment) { { 'https_proxy' => 'http://proxy:9000' } }
+    let(:environment) { { 'https_proxy' => 'http://proxy:9000', 'HTTPS_PROXY' => nil } }
 
     it 'uses https_proxy if specified and URL is secure' do
       stub_request(:get, uri_secure)
@@ -223,7 +223,7 @@
 
   context do
 
-    let(:environment) { { 'HTTPS_PROXY' => 'http://proxy:9000' } }
+    let(:environment) { { 'HTTPS_PROXY' => 'http://proxy:9000', 'https_proxy' => nil } }
 
     it 'uses HTTPS_PROXY if specified and URL is secure' do
       stub_request(:get, uri_secure)