HttpPluginServlet: Use try-with-resource in readWholeEntry

This was already done on master/stable-2.12 in change I94f481a33
which updated several places to use try-with-resource.

Backporting this specific one so it's easier to apply Luca's
fix for "short read of block" without conflict.

Change-Id: I1ec84f15b09b860c3f416a5000ad4e950a2a3977
1 file changed