Change write_write method signatures

[ggodlewski]

I was trying to achieve getting spreadsheet into memory using following code:

let mut data: Vec<u8> = Vec::new();
umya_spreadsheet::writer::xlsx::write_writer(&spreadsheet, &mut data)?;
Ok(data)

It was failing so:

• I removed unnecessary Seek trait
• and moved mut into parameter

After that it works perfectly

[MathNya]

@ggodlewski Thanks for the PR. We have confirmed that there are no problems with the revisions and have merged them.