We can't use Content-Encoding to verify if `aws-chunked` is set
or not. Just use 'streaming' signature header instead.
While this is considered mandatory, on the contrary aws-sdk-java
doesn't set this value
http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-streaming.html
```
Set the value to aws-chunked.
```
We will relax it and behave appropriately. Also this PR supports
saving custom encoding after trimming off the `aws-chunked`
parameter.
Fixes#3983
Content-Encoding is set to "aws-chunked" which is an S3 specific
API value which is no meaning for an object. This is how S3
behaves as well for a streaming signature uploaded object.
This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.
This also fixes `goconst ./...` reported issues.
Golang HTTP client automatically detects content-type but
for S3 clients this content-type might be incorrect or
might misbehave.
For example:
```
Content-Type: text/xml; charset=utf-8
```
Should be
```
Content-Type: application/xml
```
Allow this to be set properly.
