Fix unit promotion for same units, different context

[gpeairs]

Unit promotion for "same units but different promotion context" uses a unit singleton instance as a type parameter, but it should use the type.

Example:

import Unitful: μm, mm, ContextUnits
μm2μm = ContextUnits(μm,μm)
μm2mm = ContextUnits(μm,mm)
x, y = promote(1.0μm2μm, 1.0μm2mm)
upreferred(x)

Before the fix:

ERROR: MethodError: no method matching (::Unitful.FreeUnits{(m,), 𝐋, nothing})()

After the fix:

1.0e-6 m

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #845   +/-   ##
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  Coverage          ?   90.13%           
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  Files             ?       22           
  Lines             ?     1713           
  Branches          ?        0           
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  Hits              ?     1544           
  Misses            ?      169           
  Partials          ?        0           

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