Creates a new mime tree node. Assumes 'multipart/*' as the content type
if it is a branch, anything else counts as leaf. If rootNode is missing from
the options, assumes this is the root.
Adds a header value. If the value for selected key exists, the value is appended
as a new field and old one is not touched.
You can set multiple values as well by using [{key:'', value:''}] or
{key: 'value'} as the first argument.
Sets body content for current node. If the value is a string, charset is added automatically
to Content-Type (if it is text/*). If the value is a Buffer, you need to specify
the charset yourself
setHeader(key: string, value: string | string[]): this
setHeader(headers: object | Array<object>): this
Sets a header value. If the value for selected key exists, it is overwritten.
You can set multiple values as well by using [{key:'', value:''}] or
{key: 'value'} as the first argument.
Creates a new mime tree node. Assumes 'multipart/*' as the content type if it is a branch, anything else counts as leaf. If rootNode is missing from the options, assumes this is the root.