v0.2 Regression in Single-Value Quantile

[wspeirs]

There appears to be a regression in v0.2 for single-value sketches... quick testing shows the issue with any value, and any quantile:

    let mut dd = DDSketch::new(Config::defaults());

    dd.add(1234f64);

    assert_eq!(1234f64, dd.quantile(0.5).unwrap().unwrap());

The above works (produces 1234.0) in v0.1.3, but produces 1224.3764974385315 in v0.2.0/master.

[mheffner]

Thanks for filing @wspeirs. The latest version v0.2.0 is supposed to follow the Python implementation (Go too?) a bit closer, so I may take a look and see what those implementations report for single-value sketches too.