System update breaks JRuby gem shebang on Darwin

When updating RubyGems it installs a gem binscript. However the shebang for this script attempts to reference the jruby executable directly:

diff --git a/bin/gem b/bin/gem
index 37772c863f..fc75806dcc 100755
--- a/bin/gem
+++ b/bin/gem
@@ -1,4 +1,18 @@
-#!/usr/bin/env jruby
+#!/Users/headius/projects/jruby-9.2/bin/jruby

However, by default this executable is a bash script, and on Darwin (and I think on other BSDs as well) you cannot put a bash script in the shebang of another language's script. When you do, it attempts to run the language script with bash, producing errors like this:

$ gem list bundler
/Users/headius/projects/jruby-9.2/bin/gem: line 8: require: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 9: require: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 10: require: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 12: args: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 14: begin: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 15: Gem::GemRunner.new.run: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 16: rescue: command not found
/Users/headius/projects/jruby-9.2/bin/gem: line 17: exit: e.exit_code: numeric argument required

Perhaps the installation of the gem binscript should check if the executable is a script and use env instead? I am not sure what the best solution is.

Note: JRuby also has a gem jruby-launcher that when installed replaces the jruby bash script with a native binary. This executable can be put in the shebang line.

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Start with the generated bin/gem binscript and reproduce the failure on Darwin using the reported gem list bundler command with JRuby. Compare behavior when jruby is the default bash script versus the native jruby-launcher; done means the installed gem binscript no longer runs Ruby code under bash.