has_mask_band returns true for file with an alpha band

[vincentsarago]

import rasterio
from rio_cogeo import utils

with rasterio.open("https://async-cog-reader-test-data.s3.amazonaws.com/201404_13SED190110_201404_0x1500m_CL_1_alpha.tif") as src:
    print(utils.has_alpha_band(src))
    print(utils.has_mask_band(src))
    print(src.mask_flag_enums)
>>> True
>>> True
>>> ([<MaskFlags.per_dataset: 2>, <MaskFlags.alpha: 4>],
 [<MaskFlags.per_dataset: 2>, <MaskFlags.alpha: 4>],
 [<MaskFlags.per_dataset: 2>, <MaskFlags.alpha: 4>],
 [<MaskFlags.all_valid: 1>])

cc @geospatial-jeff