Made the bytes to be encoded for base64 scheme to be divisble by 3

[Gautham-coder]

…g scheme

Proposed changes

When using base64 encoding, the number bytes encoded, causes artifacts if its not divisible by 3 for files larger than 1MB. Currently, this is value is FILE_STREAM_BUFFER_SIZE(defined as 1024000). To fix this issue for the base64 condition we decrease it by 1 to make it divisible by 3.
There is also a comment added to modify this part of code incase there is a change in FILE_STREAM_BUFFER_SIZE value in the future.

This PR addresses Issue: zowe/zss#657

Type of change

• Bug fix (non-breaking change which fixes an issue)

PR Checklist

Please delete options that are not relevant.

• If the changes in this PR are meant for the next release / mainline, this PR targets the "staging" branch.
• My code follows the style guidelines of this project (see: Contributing guideline)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes
• video or image is included if visual changes are made
• Relevant update to CHANGELOG.md
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works, or describe a test method below

Testing

You can confirm by viewing a file of size greater than 1MB on the zlux-editor or get a file with 'responseType=B64'.

Further comments

[JoeNemo]

Approved.