R1:72a03dc03e7d

Add a setup warning for "always_populate_raw_post_data"

Summary:
Fixes T9235. When the stars align, PHP 5.6 or newer emits a deprecation warning on startup about "always_populate_raw_post_data" which occurs too early for us to intercept and can break responses by adding garbage to the output.

These settings appear to be sufficient:

```
always_populate_raw_post_data = 1
display_errors = 1
display_startup_errors = 1
error_reporting = -1
```

Then make a request with an unusual content type:

```
$ curl -X…