Fix unresolved issues from original repo

[stepanbenes]

• Fix issues mentioned in Support for nopcommerce 4.30  SevenSpikes/api-plugin-for-nopcommerce#223 (comment)

• Port all fixes from https://github.com/MaximG1234/api-plugin-for-nopcommerce/commits/nopCommerce_4.10

[stepanbenes]

GroupBy issue resolved in #30

[D3bo]

@stepanbenes I think we can close this historical issue? unless you remember what elements needed to be handled.

[stepanbenes]

Yes, it can be closed. I fixed most of the mentioned issues, others were irrelevant. I kept it just for reference.