### Motivation

Fixes apple/swift-openapi-generator#525.

Turns out that the mere act of creating a decoding container is
meaningful and we skipped it as an optimization, causing JSONDecoder to
fail for empty dictionaries when used in additional properties.

### Modifications

Remove the extra guards that skipped creating a container, even when we
already know there are no elements.

### Result

No more failures when encoding empty dictionaries in
additionalProperties.

### Test Plan

Tested manually as this requirement seems to be coming out of
JSONDecoder.

### Motivation

See
[apple/swift-openapi-generator#556](apple/swift-openapi-generator#556)
for more.

### Modifications

Add converter methods for encoding and decoding XML request and response
body.
Add `CustomCoder` protocol, allows to use other `Encoder` and `Decoder`
for other `content-type` body.

User can define custom coder, and assign a custom coder to a specific
content-type within `Configuration.customCoders` dictionary.

### Result

It's now possible to define custom encoder and decoder for supported
content-type.

### Test Plan

Added converter methods are tested with a mock custom coder for
`application/xml` content-type. To avoid adding a dependency to a
XMLCoder like
[CoreOffice/XMLCoder](https://github.com/CoreOffice/XMLCoder), mock
custom coder uses JSONEncoder and JSONDecoder.

Encoding and decoding to XML are out of scope of the tests, because
encoding and decoding logic must be provided by user through custom
coder implementation.

---------

Signed-off-by: Ugo Cottin <[email protected]>
Co-authored-by: Honza Dvorsky <[email protected]>

…rs (#104)

### Motivation

When the server response has a content-type header that does not conform
to the OpenAPI document, we currently throw an error. However, this
error presents itself in a very confusing manner: it prints `Unexpected
Content-Type header: application/json`, where `application/json is the
_expected_ content-type. At best, this is ambiguous and potentially
misleading.

### Modifications

- Extend `case RuntimeError.unexpectedContentTypeHeader` with _both_ the
expected and received content-type associated values.
- Update the printed description to include both the expected and
received content-type header values.

### Result

When an unexpected content-type is received, the error message is
clearer.

### Test Plan

Updated the existing test that expects an error to check the error and
that the error values are provided in the correct order.

### Motivation

The runtime changes for:
apple/swift-openapi-generator#259

### Modifications

Added `deepObject` style to serializer & parser in order to support
nested keys on query parameters.

### Result

Support nested keys on query parameters.

### Test Plan

These are just the runtime changes, tested together with generated
changes.

---------

Co-authored-by: Honza Dvorsky <[email protected]>