[rubygems/rubygems] Add support in binstubs for trampolining bundler

If `bundler _<version>_` is given, I guess the most reasonable approach
is to completely skip version switching, because the user is technically
opting out of it. But since binstubs completely remove this argument
from `ARGV` after processing it, we have no way of detecting that it was
actually passed in the first place in order to skip the feature. So we
set `BUNDLER_VERSION` explicitly in this case.

https://github.com/rubygems/rubygems/commit/e0f360d6d7

lib/rubygems/installer.rb

@@ -768,7 +768,7 @@ str = ARGV.first
 if str
   str = str.b[/\\A_(.*)_\\z/, 1]
   if str and Gem::Version.correct?(str)
-    version = str
+    #{explicit_version_requirement(spec.name)}
     ARGV.shift
   end
 end
@@ -791,6 +791,16 @@ Gem.use_gemdeps
 TEXT
   end
 
+  def explicit_version_requirement(name)
+    code = "version = str"
+    return code unless name == "bundler"
+
+    code += <<-TEXT
+
+    ENV['BUNDLER_VERSION'] = str
+TEXT
+  end
+
   ##
   # return the stub script text used to launch the true Ruby script