When rendering world space canvas to RenderTexture, alpha value of every pixel in rendered texture is affected by the canvas element which is rendered on the top of all other objects rendered by camera.

Reproduction steps:
1. Open attached project (case_847371-CanvasRednerTexture.zip )
2. Open "main" scene

Actual results:
The "RenderedTexture" game object has blue rectangle which is transparent in the middle.

Expected results:
The "RenderedTexture" game object should NOT have transparent part

See attached picture for Actual vs Expected results (blending.png)

In the scene there is "OffscreenRendering" game object which contains all RenderTexture setup. There is red transparent image in front of all other objects. Eventhough all the objects behind red image are not transparent, the rendered texture has transparency where the red image is rendered. (Visible both in Scene and Game views)
Note that alpha values on rendered texture are not exactly the same as of red image. When red image alpha is <0.5 and we decrease it, the rendered texture alpha is increasing.

Canvas rendered directly to Game/Scene view is rendered correctly. (WorldSpaceCanvas game object)

Tested and reproduced on: Unity 5.0.4f1, 5.3.3f1, 5.4.1f1, 5.4.2p3, 5.5.0b10, 5.6.0a1