nav2_costmap_2d: add missing default_value_ copy in Costmap2D operator=

[DylanDeCoeyer-Quimesis]

default_value_ is an attribute of the Costmap2D class and should be copied along with the other attributes.

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	/
Primary OS tested on	Ubuntu
Robotic platform tested on	/
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

Description of documentation updates required from your changes

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Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[SteveMacenski]

@DylanDeCoeyer-Quimesis Please sign off commits with DCO - see the failed CI job for how to do so. Then I can merge :-)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
nav2_costmap_2d/src/costmap_2d.cpp	87.55% <100.00%> (+0.05%)	⬆️

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