EVM Node: Support charset=utf-8 in Content-Type

The EVM node does not support letting the user select their charset as part of
the Content-Type definition. It is arguably a problem that we inherit from
our HTTP stack, and a proper fix should be implemented at this level.

In the meantime, we propose this dirty but working patch to support the UTF-8
charset.

Manually testing the MR

I have deployed an observer node with the patch.

Check it is the correct version

$curl https://rpc.etherlink-testnet.tz.soap.coffee/version
"octez-evm-node/Octez 0.0+dev-30fd558e/Unix/ocamlc.4.14.1"

Then try manually to call the node with a Content-Type mentioning the charset to use.

$ curl -f -X POST -H 'Content-Type: application/json; charset=utf-8' --data '{"jsonrpc":"2.0","method":"eth_call","params":[{"to":"0x37d40f4CD7c085Acaa915Ad614749236a9EF8f0B","data":"0x19ba0b63"},"latest"],"id":1}' https://rpc.etherlink-testnet.tz.soap.coffee
{"jsonrpc":"2.0","result":"0x0000000000000000000000000000000000000000000000000000000000000000","id":1}

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR