fix servo not reattaching with same target

[glmnet]

What does this implement/fix?

Fixes servo not attaching if detached and commanded with the same value again

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Configuration change (this will require users to update their yaml configuration files to keep working)

Related issue or feature (if applicable): fixes esphome/issues#1620

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP8266
• Windows
• Mac OS
• Linux

Explain your changes

Added additional call to internal_write to cover this edge case.

Describe your changes here to communicate to the maintainers why we should accept this pull request.
Very important to fill if no issue linked

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.