Fixed converting image to grayscale when image is not RGB

[RaphaelVRossi]

Fix:

OpenCV(4.5.5) 
/Users/xperience/actions-runner/_work/opencv-python/opencv-python/opencv/modules/imgproc/src/color.simd_helpers.hpp:92: 
error: (-2:Unspecified error) in function 'cv::impl::(anonymous namespace)::CvtHelper<cv::impl::(anonymous namespace)::Set<3, 4, -1>, cv::impl::(anonymous namespace)::Set<1, -1, -1>, cv::impl::(anonymous namespace)::Set<0, 2, 5>, cv::impl::(anonymous namespace)::NONE>::CvtHelper(cv::InputArray, cv::OutputArray, int) [VScn = cv::impl::(anonymous namespace)::Set<3, 4, -1>, VDcn = cv::impl::(anonymous namespace)::Set<1, -1, -1>, VDepth = cv::impl::(anonymous namespace)::Set<0, 2, 5>, sizePolicy = cv::impl::(anonymous namespace)::NONE]'
> Invalid number of channels in input image:
>     'VScn::contains(scn)'
> where
>     'scn' is 1

[coveralls]

Pull Request Test Coverage Report for Build 3840784828

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 84.852%

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Totals
Change from base Build 3800749551:	0.07%
Covered Lines:	414
Relevant Lines:	519

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💛 - Coveralls

[marcelometal]

Hi @RaphaelVRossi , please add some tests to this fix =)

[codeclimate]

Code Climate has analyzed commit 4d8aae4 and detected 0 issues on this pull request.

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[devppjr]

LGTM 🚀