Fixed Version in FormSignerV4

How to reproduce: try to assign tag to EC2 instance with string `Version` or `Action` inside.

Example reproduce:
```
o.j.h.i.JavaUrlHttpCommandExecutorService: Sending request -1118951214: POST https://ec2.us-east-1.amazonaws.com/ HTTP/1.1
jclouds.wire: >> "Action=CreateTags&Tag.1.Key=Name&Tag.1.Value=test-54886996e4b07f52e92c6416-54886996e4b0bc25677c9675&Tag.2.Key=owner_id&Tag.2.Value=522dbb16e4b0ab4eebbf6cd0&Tag.3.Key=owner_name&Tag.3.Value=Nikolay%20Sokolov&Tag.4.Key=account_id&Tag.4.Value=54886774e4b07f52e92c6366&Tag.5.Key=instance_name&Tag.5.Value=Sunset%20Dust%20Chef%20Custom%20Version&Tag.6.Key=account_name&Tag.6.Value=nsokolov-lime-tests&Tag.7.Key=application_name&Tag.7.Value=Chef%20Custom%20Version&Tag.8.Key=instance_id&Tag.8.Value=54886996e4b07f52e92c6416&Tag.9.Key=application_id&Tag.9.Value=54886804e4b07f52e92c63a2&ResourceId.1=i-55928eb4"
jclouds.headers: >> POST https://ec2.us-east-1.amazonaws.com/ HTTP/1.1
jclouds.headers: >> Host: ec2.us-east-1.amazonaws.com
jclouds.headers: >> X-Amz-Date: 20141210T154200Z
jclouds.headers: >> Authorization: AWS4-HMAC-SHA256 Credential=AKIAJAN2X6RN5VISZ3DA/20141210/us-east-1/ec2/aws4_request, SignedHeaders=content-type;host;x-amz-date, Signature=6dbf5e35c6db5febacf05ad7666661219f3912cbbbeefa4c4b790ea4f1eaaeb2
jclouds.headers: >> Content-Type: application/x-www-form-urlencoded
jclouds.headers: >> Content-Length: 603
s.n.w.p.h.HttpURLConnection: sun.net.www.MessageHeader@70896b489 pairs: {POST / HTTP/1.1: null}{X-Amz-Date: 20141210T154200Z}{Authorization: AWS4-HMAC-SHA256 Credential=AKIAJAN2X6RN5VISZ3DA/20141210/us-east-1/ec2/aws4_request, SignedHeaders=content-type;host;x-amz-date, Signature=6dbf5e35c6db5febacf05ad7666661219f3912cbbbeefa4c4b790ea4f1eaaeb2}{User-Agent: jclouds/1.7.4-SNAPSHOT java/1.7.0_65}{Content-Type: application/x-www-form-urlencoded}{Host: ec2.us-east-1.amazonaws.com}{Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2}{Connection: keep-alive}{Content-Length: 603}
s.n.w.p.h.HttpURLConnection: sun.net.www.MessageHeader@4de27db05 pairs: {null: HTTP/1.1 400 Bad Request}{Transfer-Encoding: chunked}{Date: Wed, 10 Dec 2014 15:41:59 GMT}{Cneonction: close}{Server: AmazonEC2}
o.j.h.i.JavaUrlHttpCommandExecutorService: Receiving response -1118951214: HTTP/1.1 400 Bad Request
jclouds.headers: << HTTP/1.1 400 Bad Request
jclouds.headers: << Date: Wed, 10 Dec 2014 15:41:59 GMT
jclouds.headers: << Transfer-Encoding: chunked
jclouds.headers: << Server: AmazonEC2
jclouds.headers: << Cneonction: close
jclouds.headers: << Content-Type: application/unknown
jclouds.wire: << "<?xml version="1.0" encoding="UTF-8"?>[\n]"
jclouds.wire: << "<Response><Errors><Error><Code>InvalidAction</Code><Message>The action CreateTags is not valid for this web service.</Message></Error></Errors><RequestID>41008176-6d86-4a80-bdc3-cbc8e220c732</RequestID></Response>"
```
Assert that the parameter has the expected value, just for completeness?