Fix code formatting

[ccostin93]

• use standard HttpStatus constants
• add Nonnull annotations
• fix some code warnings

[codecov]

Codecov Report

Merging #326 into master will decrease coverage by 0.04%.
The diff coverage is 44.44%.

@@             Coverage Diff              @@
##             master     #326      +/-   ##
============================================
- Coverage     82.95%   82.90%   -0.05%     
  Complexity      966      966              
============================================
  Files           169      169              
  Lines          2540     2533       -7     
  Branches         21       21              
============================================
- Hits           2107     2100       -7     
  Misses          414      414              
  Partials         19       19              

Impacted Files	Coverage Δ	Complexity Δ
...org/proshin/finapi/accesstoken/FpAccessTokens.java	96.15% <ø> (-0.52%)	7.00 <0.00> (ø)
...oshin/finapi/account/in/DailyBalancesCriteria.java	100.00% <ø> (ø)	10.00 <0.00> (ø)
...org/proshin/finapi/account/in/FpQueryCriteria.java	100.00% <ø> (ø)	11.00 <0.00> (ø)
...java/org/proshin/finapi/bank/in/BanksCriteria.java	92.85% <ø> (ø)	13.00 <0.00> (ø)
...roshin/finapi/bankconnection/FpBankConnection.java	69.76% <ø> (ø)	18.00 <0.00> (ø)
.../proshin/finapi/category/in/CashFlowsCriteria.java	100.00% <ø> (ø)	22.00 <0.00> (ø)
...proshin/finapi/category/in/CategoriesCriteria.java	100.00% <ø> (ø)	9.00 <0.00> (ø)
...org/proshin/finapi/client/out/FpConfiguration.java	86.95% <ø> (ø)	17.00 <0.00> (ø)
...ain/java/org/proshin/finapi/endpoint/Endpoint.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
...n/java/org/proshin/finapi/endpoint/FpEndpoint.java	66.66% <0.00%> (-0.35%)	12.00 <0.00> (ø)
... and 18 more

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[sonarcloud]

SonarCloud Quality Gate failed.

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
1 Code Smell

66.7% Coverage
0.0% Duplication

The version of Java (1.8.0_252) you have used to run this analysis is deprecated and we will stop accepting it from October 2020. Please update to at least Java 11.
Read more here