While setting the content-encoding to gzip in the http header manager and trying to send the POST request, the http body content isn't compressed and eventually throws an error. 

Would be good to add support for gzip, deflate for the request too. Looked at the code and we do use GzipInputStream for the responses. 

http://svn.apache.org/repos/asf/jmeter/trunk/src/protocol/http/org/apache/jmeter/protocol/http/sampler/HTTPHC3Impl.java
<snip>
if (instream != null) {// will be null for HEAD
                instream = new CountingInputStream(instream);
                try {
                    Header responseHeader = httpMethod.getResponseHeader(HTTPConstants.HEADER_CONTENT_ENCODING);
                    if (responseHeader!= null && HTTPConstants.ENCODING_GZIP.equals(responseHeader.getValue())) {
                        InputStream tmpInput = new GZIPInputStream(instream); // tmp inputstream needs to have a good counting
                        res.setResponseData(readResponse(res, tmpInput, (int) httpMethod.getResponseContentLength()));                        
                    } else {
                        res.setResponseData(readResponse(res, instream, (int) httpMethod.getResponseContentLength()));
                    }
                } finally {
                    JOrphanUtils.closeQuietly(instream);
                }
            }
<snip>