Properties such as 'spring.profiles.active' cannot be specified at the
command line under Bash and other shells due to variable naming
constraints. This change allows for exchanging underscores for periods
as well as capitalizing property names for more idiomatic naming when
dealing with environment variables.
For example, Spring will respect equally either of the following:
spring.profiles.active=p1 java -classpath ... MyApp
SPRING_PROFILES_ACTIVE=p1 java -classpath ... MyApp
The former is not possible under Bash, while the latter is. No code or
configuration changes are required; SystemEnvironmentPropertySource
adapts for these varations automatically.
SystemEnvironmentPropertySource is added by default as
"systemEnvironment" to StandardEnvironment and all subtypes, taking the
place of the plain MapPropertySource that was in use before this change.
Issue: SPR-8869
143db0d8de