Fix sanitize options function in hackney adapter converter

[riverrun]

Background

When setting ssl options in hackney, we need to get the existing ssl options and merge them with any needed additions. The problem is that the existing ssl options contain function names, which the hackney converter sanitize_options function does not properly handle, resulting in an ArgumentError.

Solution

Updated the hackney converter sanitize_options function.

[parroty]

Thank you!