Request headers

all request headers are optional

`accept-encoding` - `array` or `string`

the accept-encoding header is used to specify the encoding that the client can accept
it is a list of encodings, in order of preference
if it is a string, it is a single encoding
if it is an array, it is a list of encodings
if the server does not support the specified encoding, it should respond with a 412 Precondition Failed status code
valid encodings are gzip, deflate, br, identity (no encoding)

`accept-language` - `array` or `string`

the accept-language header is used to specify the language that the client can accept
much like the accept-encoding header, it is a list of languages, in order of preference
if the server does not support the specified language, it should respond with a 406 Not Acceptable status code
valid languages are ISO 639-1 language codes, followed by a hyphen, and then a valid ISO 3166 Alpha 2 country code
e.g. en-US, fr-CA

`authorization` - `string`

the authorization header is used to specify the credentials that the client is using to authenticate itself
the value is a string, the exact format of which is determined by the server's configuration

`cookies` - `object`

the cookies header is used to specify the cookies that the client has
it is an object, where the keys are the names of the cookies, and the values are the cookies themselves
e.g. {"cookie1": "value1", "cookie2": "value2"}
if they are in an invalid format, the server will respond with a 400 Bad Request status code

`if-modified-since` - `string`

the if-modified-since header is used to tell the server to only send the message if it has been modified since the specified date
the value is a string, in the format %Y-%m-%dT%H:%M:%SZ%z
if it has not been modified, the server will respond with a 304 Not Modified status code

`if-unmodified-since` - `string`

the if-unmodified-since header is used to tell the server to only send the message if it has not been modified since the specified date
the value is a string, in the format %Y-%m-%dT%H:%M:%SZ%z
if it has been modified, the server will respond with a 412 Precondition Failed status code

`expect` - `string` (only `100-continue` is supported)

the expect header is used to tell the server to only send the message if the specified condition is met
the only supported value is 100-continue
if the server does not support the specified condition, it should respond with a 501 Not Implemented status code

`ignore-invalid-headers` - `boolean`

the ignore-invalid-headers header is used to tell the server to ignore any headers that it does not understand
if set to true, the server will ignore any headers that it does not understand
if set to false, the server will respond with a 400 Bad Request status code if it encounters any headers that it does not understand
should be false by default