Fallback from chunked to regular uploader for small files

### Is your feature request related to a problem? Please describe.
When I run the chunked uploader on a too-small file size, I get the following:

```
boxsdk.exception.BoxAPIException: Message: File size 302023 less than minimum allowed for this API: 20000000
Status: 400
Code: file_size_too_small
```

### Describe the solution you'd like
Allow a generic upload function that falls back to the default uploading if chunked uploader refuses the file.

I realize this might be a bit high level, but considering how smooth most of the SDK is (thank you!) I feel like the folder upload (`folder.upload(path)`) is so clean and simple it would be a simple fallback for the chunked uploader.

### Describe alternatives you've considered
The alternative is a file size check, two code paths, etc.

Start by tracing the chunked uploader and the folder.upload(path) entry point described in the issue, then compare how each handles files below the chunked-upload minimum. Define the generic upload behavior and its completed-upload expectations before implementing it, and verify both small-file fallback and regular chunked uploads with tests.

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