Add serialization for JSON and form bodies in post

[wingertge]

Description

Allows for passing JSON payloads and form data in the shape of a record or a list of key value pairs (only for forms to allow duplicates). Existing behaviour is preserved as it previously only accepted strings or binary data and it parses --content-type to distinguish between JSON and form bodies.

Examples

post -t application/json url.com { test: abcd }
post -t application/x-www-form-urlencoded [ test abcd test efgh ]

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

[fdncred]

Thanks!