fix(providers): enforce lowercase headers, enforce categories

[bodinsamuel]

Changes

• enforce lowercase headers
  Headers are case insensitive, but that's annoying when we manipulate those strings against axios/fetch that always lowercase headers
• enforce categories name

[TBonnin]

fine by me. Are we sure no logic relies on the case though?

[bodinsamuel]

fine by me. Are we sure no logic relies on the case though?

It's in reaction of #3108 this is the only place those headers are used (and proxy obv)

[nalanj]

🙌🏻 I bumped into header casing on this testing PR I've been working on.