go.tools/playground: parse shebang correctly
go.tools/playground: parse shebang correctly

Fixes issue 7321.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/63560043

d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,8limbs/gotools,d0f/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools