Assertion on content-length mismatch
Reproduce with

```
import options, asyncdispatch

import httpbeast

proc onRequest(req: Request): Future[void] =
  if req.httpMethod == some(HttpPost):
    case req.path.get()
    of "/":
      req.send("Hello World")
    else:
      req.send(Http404)

run(onRequest)
```

Then send a faulty HTTP request with:

```
curl -v -XPOST http://127.0.0.1:8080/ -d/etc/services -H "Content-Length: 12"
```

Results in:

```
httpbeast.nim(421)       run
httpbeast.nim(414)       run
httpbeast.nim(287)       eventLoop
httpbeast.nim(203)       processEvents
httpbeast.nim(140)       bodyInTransit
system.nim(3877)         failedAssertImpl
system.nim(3870)         raiseAssert
system.nim(2916)         sysFatal
Error: unhandled exception: /home/idoornekamp/.nimble/pkgs/httpbeast-0.2.1/httpbeast.nim(140, 9) `not
  trueLen < bodyLen`  [AssertionError]```
```

IMHO asserting or throwing on faulty remote data is not right here.