fix: resolve duplex error when fetching with stream body

[banyudu]

Without this patch, fetch throws

TypeError: Failed to execute 'fetch' on 'Window':
 The `duplex` member must be specified for a request with a streaming body

when running with a stream body.

fetch(new Request('example1.txt', { method: 'POST' }))  // => ok
fetch(new Request('example1.txt', { method: 'POST', body: '{}' }))  // => exception

This patch converts the stream body to plain text before sending it to native fetch.

[TrickyPi]

Thanks!