[Go][Client]Go onoef discriminator err

Created by: code-lucidal58

most of the commits are updates from master.

Only two commits are of use:

https://github.com/OpenAPITools/openapi-generator/commit/9705bbe4d0bfa958be8213b2f43dc147b487ecc7 https://github.com/OpenAPITools/openapi-generator/commit/be0a53f672e44ac51b49dc488c11c3326756512e

Here is the logic I have tried to implement

if useOneOfDiscriminatorLookup and discriminator:
  use discriminator to marshal
else:
  use object marshalling and check if it fits into any struct

Since mustache does not have and operator, there is a block of code that is used twice.

if useOneOfDiscriminatorLookup:
  if discriminator:
    use discriminator to marshal
  else:
    use object marshaling and check if fits into any struct
else:
  use object marshaling and check if fits into any struct

@antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)