fastcgi: Fix Caddyfile parsing when handle_response is used

[francislavoie]

Fixes #4322

Super simple, actually. The problem was that if handle_response contained a root directive, then it would actually get consumed ahead of time as a subdirective for php_fastcgi, breaking the tokens, triggering an error when parsing when passed down to reverse_proxy.

The fix is to only read php_fastcgi subdirectives if the nesting is 1, so we skip any that might appear within any sub-blocks.

The test fails before the change, passes after.

[mholt]

Fascinating. Thanks for the elegant fix and test case, too!