Reproduction steps:
1. Open the attached project " Test_DoesSourceTextureHaveAlpha.zip";
2. In the Assets, go to the Graphics folder;
3. Find tex_Eawase_00.png and reimport it;
4. Then find text_tap_00.png and reimport it;
5. Observe the results in the console window;

Note: tex_Eawase_00.png has no Alpha channel, only text_tap_00.png has.

Expected results: this function has to show does textures source image have an alpha channel.
Actual results: the function provides answer TRUE even if the texture does not have an alpha channel.

Reproduce on: 5.5.1p1, 5.6.0b7.
Not reproduced on: 5.4.2p3.