Fix incompatibility with zsh (mkdir error)

* Unlike bash, zsh creates undeclared parameters on export
* Exporting undeclared GEM_HOME causes gem env gemdir to return blank
* rvm_ruby_gem_home is then set blank
* mkdir -p "$rvm_ruby_gem_home" (gemset_select) fails

scripts/cli

@@ -643,7 +643,7 @@ rvm()
 
   rvm_action="${rvm_action:-usage}"
 
-  export BUNDLE_PATH GEM_HOME GEM_PATH rvm_action rvm_archflags rvm_bin_flag rvm_bin_path rvm_clang_flag rvm_configure_flags rvm_debug_flag rvm_delete_flag rvm_docs_type rvm_dump_environment_flag rvm_error_message rvm_file_name rvm_gemdir_flag rvm_gemset_name rvm_head_flag rvm_install_arguments rvm_install_on_use_flag rvm_interactive_flag rvm_llvm_flag rvm_loaded_flag rvm_make_flags rvm_niceness rvm_only_path_flag rvm_parse_break rvm_patch_names rvm_patch_original_pwd rvm_pretty_print_flag rvm_prior_cc rvm_proxy rvm_quiet_flag rvm_ree_options rvm_reload_flag rvm_remove_flag rvm_ruby_alias rvm_ruby_aliases rvm_ruby_args rvm_ruby_file rvm_ruby_gem_home rvm_ruby_interpreter rvm_ruby_load_path rvm_ruby_make rvm_ruby_make_install rvm_ruby_patch_level rvm_ruby_repo_url rvm_ruby_require rvm_ruby_string rvm_ruby_strings rvm_ruby_version rvm_script_name rvm_sdk rvm_silent_flag rvm_system_flag rvm_token rvm_trace_flag rvm_use_flag rvm_user_flag rvm_verbose_flag rvm_wrapper_name
+  __rvm_export_if_set BUNDLE_PATH GEM_HOME GEM_PATH rvm_action rvm_archflags rvm_bin_flag rvm_bin_path rvm_clang_flag rvm_configure_flags rvm_debug_flag rvm_delete_flag rvm_docs_type rvm_dump_environment_flag rvm_error_message rvm_file_name rvm_gemdir_flag rvm_gemset_name rvm_head_flag rvm_install_arguments rvm_install_on_use_flag rvm_interactive_flag rvm_llvm_flag rvm_loaded_flag rvm_make_flags rvm_niceness rvm_only_path_flag rvm_parse_break rvm_patch_names rvm_patch_original_pwd rvm_pretty_print_flag rvm_prior_cc rvm_proxy rvm_quiet_flag rvm_ree_options rvm_reload_flag rvm_remove_flag rvm_ruby_alias rvm_ruby_aliases rvm_ruby_args rvm_ruby_file rvm_ruby_gem_home rvm_ruby_interpreter rvm_ruby_load_path rvm_ruby_make rvm_ruby_make_install rvm_ruby_patch_level rvm_ruby_repo_url rvm_ruby_require rvm_ruby_string rvm_ruby_strings rvm_ruby_version rvm_script_name rvm_sdk rvm_silent_flag rvm_system_flag rvm_token rvm_trace_flag rvm_use_flag rvm_user_flag rvm_verbose_flag rvm_wrapper_name
 
   case "$rvm_action" in
     use)             __rvm_use        ;;

scripts/selector

@@ -4,7 +4,7 @@
 __rvm_select()
 {
   # Set Variable Defaults
-  export GEM_HOME GEM_PATH BUNDLE_PATH MY_RUBY_HOME RUBY_VERSION
+  __rvm_export_if_set GEM_HOME GEM_PATH BUNDLE_PATH MY_RUBY_HOME RUBY_VERSION
 
   if [[ -z "${rvm_ruby_string:-""}" ]] ; then
     [[ -n "${rvm_ruby_interpreter:-""}" ]] && rvm_ruby_string="$rvm_ruby_interpreter"

scripts/utility

@@ -158,6 +158,17 @@ __rvm_quote_args()
   return 0
 }
 
+__rvm_export_if_set()
+{
+  for param_name in $*
+  do
+    eval param_value=\${"$param_name"-"__unset__"}
+    if [[ "$param_value" != "__unset__" ]]; then
+      export $1
+    fi
+  done
+}
+
 __rvm_quote_args_with_shift()
 {
   local shift_value="$1"; shift