Fixed prop types on MessageCard image width and height to avoid console error

[jakubjanczyk]

Problem/Feature

MessageCard image's width and height can be either number or string. The code is prepared for this, but PropTypes have not been allowing numbers. This PR changes PropTypes to allow both.

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Pull Request Test Coverage Report for Build 3569

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.191%

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Totals
Change from base Build 3566:	0.0%
Covered Lines:	6589
Relevant Lines:	6832

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💛 - Coveralls

[plbabin]

looks good, let's release that only when another PR is merged in master, no need to create a new npm release just for that!